Async task is blocking the UI

Question

Hello esteemed colleagues, I need to save to a text file every time I get a new message from my serial port into my RichTextBox. I'm trying to use async and await it won't block the UI while saving the text file, but unfortunately I'm doing something wrong because it still blocking. Please, what am I doing wrong?

TextChanged event handler from RichTextBox:

private async void rtb_msg_TextChanged(object sender, EventArgs e)
{
    btn_save.Enabled = true;
    rtb_msg.ScrollToCaret();
    rcvFlag = true;
    await SaveFile();
}

Async SaveFile method:

private async Task SaveFile()
{
    if (_serialPort.BytesToRead == 0 && rcvFlag == true)
    {
        for(int i = 0; i < 10; i++)
        {
            Thread.Sleep(1000);
        }
        using (StreamWriter writer = new StreamWriter(reportFolder + reportFile))
        {
            await writer.WriteAsync(rtb_msg.Text);
        }
        rcvFlag = false;
    }
}

Answer 1

This part will definitely block your UI thread:

for(int i = 0; i < 10; i++)
{
    Thread.Sleep(1000);
}

Not sure why it's there, but if you replace it with an async version, it should work:

for(int i = 0; i < 10; i++)
{
    await Task.Delay(1000);
}